The present invention relates to a joint in a high-voltage cable with electric conductors surrounded by solid insulation and the insulation of each conductor and joint being surrounded by a conducting screen, and the insulation at the joint completely or over part of its length having a coating with a resistivity which is lower than that of the conductor insulation.
In a joint in a cable where the insulation of the conductor is surrounded by a conducting screen, this screen normally is removed from the outer end of the insulation of the conductor. At the edge of the screen, the electric field intensity in the insulation becomes so high that the insulation may be damaged. It is customary that the joint in the insulation of the conductor be conically tapered with the narrow end directed towards the joint in the conductor. In a joint with this shaping, no harmful field intensity occurs in the insulation of the conductor. However, carrying out the conical termination of the solid insulation is rather difficult and demands considerable effort and time. Furthermore, it is conventional that the jointing sleeve in the connection between the conductors be wrapped with ribbons so that the outer surface of the wrapping is even and smooth. The wrapping consists of a material, the resistivity of which is lower than that of the insulation of the conductor. The jointing sleeve of metal unavoidably has some sharp corners which give rise to an increased field intensity which is detrimental to the surrounding insulation. The wrapping with conducting material assumes on its surface the same electric potential as the conductor of the cable, and, as it is even and smooth, there are no points on the surface where the field intensity can be harmful.
Swedish patent No. 164,598 shows a composition for a known compound which prevents a detrimentally high field intensity in an insulation material from originating. The beneficial effect of the compound is due to the fact that it has a voltage dependent resistivity which is always lower than that of the insulation of the conductor. However, it has not been possible so far, not even if the joint has contained said compound, to avoid the work of forming the conductor insulation into a cone.